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This paper proposes a selective harmonic voltage control approach for the static synchronous compensator (STATCOM) deployed in wind power plants (WPPs). The method enables STATCOM to mitigate voltage harmonics at the point of interconnection of WPP without using the high-voltage sensor. Further, a passivity-based design of the proposed control method is developed to eliminate the negative resistances around the concerned harmonic components, which allows robust harmonic voltage damping under the different passive grid impedances. The effectiveness of the approach is elaborated by theoretical analysis and validated by electromagnetic simulations.

This paper presents a comparative analysis of a static synchronous compensator (STATCOM) based on battery energy storage system with grid-following and grid-forming operations utilized for stability enhancement of offshore wind power plants (WPPs). The small-signal model of a typical offshore WPP system is developed first, then compared from two perspectives – oscillation index for stability margin evaluation of a multi-input multi-output system and damping ratio of dominant poles, with respect to the critical control parameters. The theoretical comparative analysis proves that the grid-forming control offers evident stability margin and damping to the WPPs especially in weak grids, superior to the grid-following STATCOM. This conclusion holds true for most conditions with properly designed parameters, including different bandwidths of the alternating voltage control of STATCOM in 1∼10 Hz and phase-locked loop of grid-following STATCOM ranging from 5 to 100 Hz, which is benefited from the sufficient stability margin of GFM mode. The small-signal models and comparative analysis are corroborated by electromagnetic transients (EMT) simulations.

This article proposes a novel capacitor-voltage balancing strategy for the double-Y modular multilevel converter static synchronous compensator operated under unbalanced current conditions of the system. The proposed method, based on zero-sequence voltage injection, manipulates the current distribution between the converter arms to minimize the converter ratings. The proposed balancing strategy is compared with the classical approach, based on controlling the circulating dc currents. For the same unbalanced conditions, the proposed strategy leads to lower energy rating for the converter as compared with the classical one, whereas the semiconductor ratings remain the same in both strategies. Theoretical findings are verified through experimental results.

With the increasing deployment of offshore wind power plants (WPPs), the grid-forming (GFM) battery energy storage system (BESS) has recently emerged as an attractive solution to improve the dynamic performances of WPPs. However, the control interactions of the GFM-BESS and offshore WPP, under different grid strengths, tend to complicate the controller-parameter tuning. This paper presents a modeling method for analyzing control interactions of offshore WPP and GFM-BESS, which sheds clear insights into the critical controller parameters to the system dynamics. Differing from conventional methods, a frequency-domain model of GFM-BESS, obtained by taking the Laplace transform of the corresponding state-space model, is developed first. This allows the impedance model of offshore WPP, including a black-box model of long transmission cable, to be flexibly integrated. Based on the model, both closed-loop transfer-function and pole-based dynamic analyses are then performed. Electromagnetic transient simulations corroborate the effectiveness of the model and analysis.

The share of renewables in the power system is increasing rapidly. Large offshore wind power plants (OWPPs) are developed at a high pace and conventional fossil fuel-based plants are decommissioned. If the OWPP gets islanded due to any contingency or in the event of a blackout, the whole OWPP will be shutdown. This paper proposes a STATCOM with a battery storage that is located at the point of common connection to an OWPP to enable OWPP energization from a fully discharged state to operate in islanded mode. The STATCOM functionality provides fast and dynamic reactive power management and the battery unit provides active power balancing capability to regulate the frequency in the island. The concept is demonstrated through time-domain simulations on an OWPP model in PSCAD. The results confirm the technical feasibility of the system.

The share of renewables in the power system is increasing rapidly. Large offshore wind power plants (OWPPs) are developed at a high pace and conventional fossil fuel-based plants are decommissioned. Consequently, there will be a risk of insufficient amount of power plants providing black start functionality for system restoration after a black out. This paper proposes a STATCOM with a battery energy storage that is located at the point of common connection to an OWPP that together can provide a reliable black start service to the power grid. The concept is demonstrated by using time domain simulations in PSCAD. The STATCOM functionality provides fast and dynamic reactive power management and the battery unit provides active power balancing capability to maintain the frequency within a tolerable range specified by the system operator. The simulation results fulfill the success criteria for the black start and confirm its feasibility for practical implementation.

This paper proposes a deep-learning-based method for frequency-dependent grid impedance estimation. Through measurement of voltages and currents at a specific system bus, the estimate of the grid impedance was obtained by first extracting the sequences of the time-dependent features for the measured data using a long short-term memory autoencoder (LSTM-AE) followed by a random forest (RF) regression method to find the nonlinear map function between extracted features and the corresponding grid impedance for a wide range of frequencies. The method was trained via simulation by using time-series measurements (i.e., voltage and current) for different system parameters and verified through several case studies. The obtained results show that: (1) extracting the time-dependent features of the voltage/current data improves the performance of the RF regression method, (2) the RF regression method is robust and allows grid impedance estimation within 1.5 grid cycles, (3) the proposed method can effectively estimate the grid impedance both in steady state and in case of large transients like electrical faults.

The original online version of this article was revised because a number of authors had the wrong affiliation number next to their names. © 2021, Springer Nature B.V.

With the increasing of wind power generation, it is of great importance to fully understand the small-signal performance of the wind power plants (WPPs). To deal with this issue, this paper developed an aggregate dq-frame impedance model for the large-scale offshore WPP, which capture the multiple timescale dynamic of the wind turbine (WT) converters. The long transmission cable is modeled by frequency scan and vector fitting method. Then, the impact of working point on the PLL, power control loop and voltage support are researched by the impedance-based small signal stability analysis, and it is shown that as the active power increase, the stability of offshore WPP will be more sensitive to PLL bandwidth but less sensitive to the voltage support parameters. The model and analysis are validated by PSCAD/EMTDC simulation.

With the growing integration of renewable resources, more and more grid-forming (GFM) converters are expected to provide voltage and frequency support with emulated inertia to the power systems. However, the small-signal stability analysis of GFM converter connecting to a complicate power network is still challenging. This paper presents an effective and general integration method for stability evaluation of GFM converters interacting with grids in different system configuration. The impedance models of all facilities are efficiently organized and integrated with GFM converters. The eigenvalues-based analysis and closed-loop input-output dynamic models considering the whole power system can be realized. Hence, the proposed method enables a more flexible and comprehensive stability evaluation tool for power systems with GFM converters. A practical case in PSCAD/EMTDC is used to prove the method.

The aim of this paper is to investigate the impact of steady-state deviations in the grid-frequency on the performance of grid-forming converter control strategies. In particular, the virtual synchronous machine (VSM) control structure has been the focus of study in this paper. Two alternative solutions to address the issue with the conventional VSM structure are suggested, and their impact on the active power response and the power dissipation properties of the converter are compared. For the latter, the ac-side input admittance of the grid-connected converter is derived for the different control structures under consideration. Finally, the dynamic performance of the different control structures is verified through the time-domain simulations.

A controller for a hybrid converter for STATCOM applications, based on a cascaded H-bridge star in series with a generic converter connected at the star point, is proposed. The controller utilizes the generic converter in an optimized way to maximize the operating range of the converter, with focus on unbalanced operating conditions. So for each operating condition, the generic converter reference voltage minimizes the voltage rating of the star part. Theoretical investigations of the hybrid converter using the proposed control approach show that the hybrid converter allows to effectively extend the operating range of the converter under unbalanced conditions of the system, given that the generic converter is large enough to circulate the needed active power between the converter phase legs. This indicates that, depending on the requirements from the operator in terms of negative-sequence injection capability, the hybrid converter can be a valuable solution to overcome the limitations of the CHB star.

This paper proposes a modular multilevel converter (MMC) topology, designed particularly for load balancing in 50 Hz railway applications. The asymmetrical character of the traction load requires higher device rating for a Double-star or Delta-connected MMC compared to a symmetrical static compensator. This is due to the symmetrizing components to rebalance the capacitor voltages in the submodules. In this paper a reduced chain-link MMC topology, consisting of a two phase-leg converter, is proposed in order to avoid overrating the semiconductors in the main current path. In addition, the main circuit is equipped with dc-dc dual half-bridge (DHB) converters that connect the submodule capacitors of the two legs. The DHBs are operating as “Power Channels,” enabling energy exchange to restore the power imbalance among the capacitors. Simulation results show that the proposed converter is capable of achieving load balancing under any operating conditions, while the balance of the capacitor voltage is ensured.

This paper evaluates three alternative topologies for a static compensator (StatCom), suitable to eliminate the negative-sequence currents in a railway system. The compensator is placed in a transformer substation, which feeds two separate sections of the catenary. The major requirements of this converter is to: (i) symmetrize the railway load as seen from the high-voltage side, (ii) control the catenary voltage to a defined level, and (iii) compensate for the harmonics generated by the rolling stock. The topologies investigated here are: (a) a 2-level converter with a common dc-bus capacitor, (b) a delta-connected modular multilevel converter (MMC) based on full-bridge submodules, and (c) a double-star MMC based on half-bridge submodules. The evaluation is based on the minimum device-rating requirements of each converter that fulfill the application requirements in the steady-state. Simulation results are provided to support the conclusions of this evaluation.

This paper investigates issues related to practical implementation of the one-quarter of cycle-based estimation method [here called delayed signal cancellation (DSC)] for online detection of positive and negative sequence components of three-phase quantities. The detection error due to non-ideal sampling frequency is calculated. Furthermore, the effect of the DSC method on estimation when harmonic components are present in the input signal is analyzed. It is shown how harmonics transfer through the DSC and how a non-ideal sampling frequency affects the estimation of the harmonics. Experimental verification of the analytical results is presented.

In this paper, a novel control strategy for subsynchronous resonance (SSR) mitigation using a static synchronous series compensator (SSSC) will be presented. The SSSC is constituted by three single-phase voltage source converters. SSR mitigation is obtained by increasing the network damping only at those frequencies that are critical for the turbine-generator shaft. This is achieved by controlling the subsynchronous component of the grid current to zero. Using the IEEE First benchmark model, the effectiveness of the proposed control algorithm when mitigating SSR due to torsional interaction and torque amplification effect will be shown.

The full potential of biologically optimized radiation therapy can only be maximized with the prediction of individual patient radiosensitivity prior to treatment. Unfortunately, the available biological parameters, derived from clinical trials, reflect an average radiosensitivity of the examined populations. In the present study, a breast cancer patient of stage I-II with positive lymph nodes was chosen in order to analyse the effect of the variation of individual radiosensitivity on the optimal dose distribution. Thus, deviations from the average biological parameters, describing tumour, heart and lung response, were introduced covering the range of patient radiosensitivity reported in the literature. Two treatment configurations of three and seven biologically optimized intensity-modulated beams were employed. The different dose distributions were analysed using biological and physical parameters such as the complication-free tumour control probability (P(+)), the biologically effective uniform dose (D), dose volume histograms, mean doses, standard deviations, maximum and minimum doses. In the three-beam plan, the difference in P(+) between the optimal dose distribution (when the individual patient radiosensitivity is known) and the reference dose distribution, which is optimal for the average patient biology, ranges up to 13.9% when varying the radiosensitivity of the target volume, up to 0.9% when varying the radiosensitivity of the heart and up to 1.3% when varying the radiosensitivity of the lung. Similarly, in the seven-beam plan, the differences in P(+) are up to 13.1% for the target, up to 1.6% for the heart and up to 0.9% for the left lung. When the radiosensitivity of the most important tissues in breast cancer radiation therapy was simultaneously changed, the maximum gain in outcome was as high as 7.7%. The impact of the dose-response uncertainties on the treatment outcome was clinically insignificant for the majority of the simulated patients. However, the jump from generalized to individualized radiation therapy may significantly increase the therapeutic window for patients with extreme radio sensitivity or radioresistance, provided that these are identified. Even for radiosensitive patients a simple treatment technique is sufficient to maximize the outcome, since no significant benefits were obtained with a more complex technique using seven intensity-modulated beams portals.

In this paper, a novel control strategy for subsynchronous resonance (SSR) mitigation using a static synchronous series compensator will be presented. SSR mitigation is obtained by increasing the network damping only at those frequencies that are critical for the turbine-generator shaft. This is achieved by controlling the subsynchronous component of the grid current to zero. Using the IEEE first benchmark model, the effectiveness of the proposed control algorithm when mitigating SSR due to torsional interaction and torque amplification effect will be shown.

In this paper, the problem of online estimation of subsynchronous frequency components in the measured grid voltage is treated. Two estimation methods, one based on the use of lowpass filters and a recursive least square algorithm, are investigated and compared. In particular, due to its higher degree of freedom in the design, the lowpass filters-based estimation method is found to be the most appropriate and accordingly further analyzed. The method is improved to cope with inaccurate knowledge of the subsynchronous frequency. The simulation results prove the effectiveness of the investigated method both for the ideal and the disturbed case.

In this paper, the use of a method for online detection of positive- and negative-sequence components of three-phase quantities, named the delayed signal cancellation (DSC) method, is investigated. Problems that arise in practical implementation of the DSC method in computer-controlled systems are investigated. Expressions of the detection error due to nonideal discretization are derived and calculations are verified experimentally. Two methods for reducing the detection error are presented and verified. It is also shown that the given expressions and proposed methods for reducing the detection errors can also be applied to the case of grid frequency variations

We summarize the inflight performance of JEM-X, the X-ray monitor on the INTEGRAL mission during the initial ten months of operations. The JEM-X instruments have now been tuned to stable operational conditions. The performance is found to be close to the pre-launch expectations. The ground calibrations and the inflight calibration data permit to determine the instruments characteristics to fully support the scientific data analysis. Reglero Velasco, Victor, Victor.Reglero@uv.es ; Martinez Nuñez, Silvia, Silvia.Martinez@uv.es

Energy storage will gain importance in the future power system due to increased amount of renewables. In this paper, a utility scale Battery Energy Storage System (BESS) for megawatt utilizing a high voltage and a high power Voltage Source Converter (VSC) is proposed to be able to inject/consume the required power and energy to the grid. The batteries are both series and parallel connected on the DC-side of the VSC. In order to obtain high reliability, high availability and safety of the BESS, this paper proposes and analyzes a distributed Solid-State DC Breaker protecting the BESS and its batteries.

Spectroscopic measurements were made of the population of the excited states in a low-voltage, low-temperature, open caesium plasma diode. Such a device is under certain conditions seen to run in a special mode that is essentially different from the ordinary plasma mode. This mode, which has been called the Rydberg current mode, exhibits many interesting features, such as a very strong electron emission from the cold to the hot electrode. The objective of the present study was to investigate and compare the plasma conditions of the two modes. The spectral line intensities and the excitation temperatures of the upper states were found to differ considerably although the current, the voltage and the electrode temperatures were comparable.

In this survey paper, applications of medium voltage direct current (MVDC) to distribution substations are discussed and compared to conventional ac for power conversion from transmission voltage levels to distribution voltage levels. Technical changes in the substation due to the dc equipment, efficiency of the substation, and protective measures are major aspects to consider in the MVDC concept development.

The concept and implementation of a tri-national EU intensive Programme (IP) is presented and discussed. The objective is to improve the methods employed by the teachers and scientist of the staff concerning the classification and management of grasslands to students and to prepare them to be able to design, analyse, and evaluate grassland management strategies and to understand grasslands in land-use-system terms. The focus lies on the development of strategies for sustainable grassland use in the context of regional development. The innovative character if this romanian-swedish-german module is exemplified.

The inheritance of 23 protein polymorphisms was compared with the inheritance of a DNA restriction fragment length polymorphism (RFLP) of a strongly cross-hybridizing erb B-related sequence, epidermal growth factor receptor-like-1 (EGFRL1), in Xiphophorus clemenciae X X. milleri-derived backcross hybrids. Two polymorphic bands were noted in this cross with a v- erb B probe after Pst I digestion: a 10-kilobase (kb) band in X. clemenciae and a 9-kb band in X. milleri. Joint segregation analysis of the RFLPs and protein polymorphisms indicate that this erb B-related sequence can be assigned to Xiphophorus linkage group VI, which also comprises genes coding for glutamine synthetase ( GLNS ), nucleoside phosphorylase-2 ( NP2 ), and transferrin ( TF ). We have designated this RFLP as alleles at a locus called EGFRL1 because of very strong cross-hybridization with the v- erb B probe, known to be homologous to the mammalian EGFR gene. This mapping assignment is the first autosomal linkage of an oncogene sequence reported in fish, which provide a large number of genetically controlled experimental tumor models.

This paper deals with the analysis and simulation of the static series compensator (SSSC) for subsynchronous resonance (SSR) mitigation. The purpose of the paper is to derive and analyze a novel control strategy for SSSC dedicated for SSR mitigation. Objective of the proposed controller is to increase the network damping only at those frequencies that are critical for the turbine-generator shaft. By using frequency scanning analysis, the effectiveness of the proposed method for mitigation of SSR due to torsional interaction effect is presented and compared with the existing control strategy. Finally, simulation results show the performance of the proposed method in mitigating SSR due to torque amplification effect.

Aims Because of the highly conformal distributions that can be obtained with intensity-modulated radiotherapy (IMRT), any discrepancy between the intended and delivered distributions would probably affect the clinical outcome. Consequently, there is a need for a measure that would quantify those differences in terms of a change in the expected clinical outcome. Materials and methods To evaluate such a measure, cancer of the cervix was used, where the bladder and rectum are proximal and partially overlapping with the internal target volume. A solid phantom simulating the pelvic anatomy was fabricated and a treatment plan was developed to deliver the prescribed dose to the phantom. The phantom was then irradiated with films positioned in several transverse planes. The racetrack microtron at 50MV was used in the treatment planning and delivery processes. The dose distribution delivered was analysed based on the film measurements and compared against the treatment plan. The differences in the measurements were evaluated using both physical and biological criteria. Whereas the physical comparison of dose distributions can assess the geometric accuracy of delivery, it does not reflect the clinical effect of any measured dose discrepancies. Results It is shown how small inaccuracies in delivered dose can affect the treatment outcome in terms of complication-free tumour cure. Conclusions With highly conformal IMRT, the accuracy of the patient set-up and treatment delivery are critical for the success of the treatment. A method is proposed to evaluate the precision of the delivered plan based on changes in complication and control rates as they relate to uncertainties in dose delivery.

We propose a new approximation for the cyclo-synchrotron emissivity of a single electron. In the second part of this work, we discuss a simple application for our approximation, and investigate the heating of electrons through the self-absorption process. Finally, we investigate the self-absorbed part of the spectrum produced by a power-law population of electrons. In comparison to earlier approximations, our formula provides a few significant advantages. Integration of the emissivity over the whole frequency range, starting from the proper minimal emitting frequency, gives the correct cooling rate for any energy particle. Further, the spectrum of the emission is well approximated over the whole frequency range, even for relatively low particle energies (beta << 0.1), where most of the power is emitted in the first harmonic. In order to test our continuous approximation, we compare it with a recently derived approximation of the first ten harmonics. Finally, our formula connects relatively smooth to the synchrotron emission at beta=0.9. We show that the self-absorption is a very efficient heating mechanism for low energy particles, independent of the shape of the particle distribution responsible for the self-absorbed synchrotron emission. We find that the energy gains for low energy particles are always higher than energy losses by cyclo-synchrotron emission. We show also that the spectral index of the self-absorbed part of the spectrum at very low frequencies differs significantly from the well known standard relation I(nu) ~ nu^(5/2)., 9 pages, 4 figures, accepted for publication in A&A

Power electronics converters connected in series with the grid can provide protection of sensitive processes against voltage dips. In this paper, an advanced cascade controller for series-connected voltage source converter (VSC), constituted by an outer voltage loop and an inner current loop, is presented. Positive and negative sequence components of voltage and current are controlled separately to achieve perfect compensation of unbalanced dips. Limitation of the VSC output voltage and compensation of the computational time delay are included in the current controller for both sequence components. Experimental results for the series-connected VSC with the proposed cascade controller under balanced and unbalanced voltage dips are presented.

The increasing use of voltage source converters (VSCs) in grid-connected applications at distribution levels requires control methods that achieve high performance not only during normal operating conditions of the grid, but also under voltage disturbances, like voltage dips. In this paper, three different current controllers for shunt-connected VSC are investigated. With a constant DC-link voltage, the three controllers are compared under balanced and unbalanced voltage dips. Simulation and experimental results are shown for all three controllers.